Copolyester-carbonate resins are well known commercially available materials which, due to their many advantageous properties, are finding increasing use as thermoplastic engineering materials. Such copolyester-carbonates may be prepared by the reaction of a dihydric phenol, a carbonate precursor, and an ester precursor. The copolyester-carbonates exhibit high heat resistance, good dimensional stability, good tensile strengths, and good impact strengths. However, in certain applications the use of aromatic copolyester-carbonate resins is limited because they exhibit environmental stress crazing and cracking. "Environmental stress crazing and cracking" refers to the type of failure which is hastened by the presence of organic solvents such as, for example, the weak organic solvents such as toluene, mineral spirits, and the like, when such solvents are in contact with stressed parts fabricated from aromatic copolyester-carbonate resins. The most significant effect is loss of vital impact strength and also an increase in brittle type failure. Contact with such solvents may occur, for example, when parts are used in or around the windshields of automobiles, since most windshield washer solutions contain some of these weak organic solvents, or when these weak organic solvents are used to clean or degrease stressed parts made from copolyester-carbonate resins.
It is an object of the instant invention to provide copolyester-carbonate resin containing compositions exhibiting improved resistance to relatively weak organic solvents, particularly those present in windshield washing solutions.